This invention pertains to using an optoelectronic sensor for inhalation therapy and inhalation monitoring, and particularly using it with such apparatus and operating methods which feature the triggering of a prescribed dose of therapeutic gas when inhalation takes place and/or detects when apnea (the absence of breathing) occurs.
The advantage of using an inhalation sensor as an apnea monitor, is that the inhalation sensor can detect obstructions in the upper air passageways immediately. Other techniques requiring electrodes to be positioned on the chest suffer from a fundamental deficiency in that they measure the thoracic effort rather than specific airway opening and patency.
The inhalation sensor, when used as an apnea monitor, overcomes this problem as it is measuring air flow at an airway opening such as the nasal openings, or if necessary, at the mouth.
A fundamental limitation of detecting inhalation with a sensor is the recognized difficulty of keeping the inhalation sensor aligned with the airway exchange.
The applicant deals with this recognized difficulty by using the nasal cannula (which is an accepted method of administering oxygen for inhalation therapy) as a means of connecting a patient to the inhalation sensor.
This optoelectronic sensor is an improvement over the device shown in the inventor's pending application, "Method and apparatus for using an inhalation sensor for monitoring and for inhalation therapy", received by Commissioner of Patents and Trademarks Feb. 20, 1986.
The optoelectronic sensor, when used with the nasal cannula for inhalation therapy, requires that the negative pressure (which can be as little as 0.001 ounce per square inch) from the nasal openings be transmitted to the sensor through the nasal cannula; that the sensor will detect this low pressure and immediately trigger a prescribed dose of therapeutic gas, using the same nasal cannula as that used for sensing the nasal negative pressure. Therefore, the sensor and cannula must alternately sense 0.001 ounce per square inch negative pressure, and withstand a pressure of 10 pounds per square inch when a dose of therapeutic gas is triggered.
In prior art, the sensing used one tube connected to one of the nasal openings, and a second tube connected to the second nasal opening for the high pressure therapeutic gas.
The optoelectronic sensor in this application requires only a single tube connected to the two prongs inserted in the patient's nasal openings to serve the sensor and supply the therapeutic gas.
The optoelectronic sensor uses an extremely thin diaphragm that is pre-stressed, and to which a very small vane is attached thereto. Very small movements of the vane are sensed by optoelectronic means.
The advantage of using optoelectronics is, if the sensor is manufactured out of opaque material it is not affected by ambient light nor by strong electric fields which create problems when capacitance type of sensors are used.
The greatest difficulty to overcome in sensors using diaphragm movement is maintaining a fixed calibration point. The diaphragm must be responsive to pressures of 0.001 ounce per square inch, yet it must be able to withstand an accidental overload of 10 pounds per square inch without change in the fixed calibration point. The calibration point is affected by the temperature changes that affect the mechanical parts and the characteristics of the electronic circuit.
When the sensor is adjusted for maximum sensitivity, the slightest change in the calibration point will cause undesired oscillation because the high pressure of the triggered dose of therapeutic gas is inadvertently being fed back into the input of the sensor causing parasitic oscillations which could mimic the breathing of the patient.
The applicant's invention deals with this difficulty by making the sensor mechanically stable and providing temperature compensation for the electronic circuit and providing a means of manual adjustment of the calibration point to overcome the parasitic oscillations.